In-Situ Combustion in General
The present invention relates to the recovery of petroleum from an underground reservoir using an in-situ forward combustion process.
`Forward combustion` is a term applied to a broad class of oilfield recovery processes in which heat is generated within the reservoir by igniting the formation oil and then propagating the combustion front, by continuous injection of an oxidizing agent such as air through an injection well (`injector`), toward an outlying production well (`producer`).
Conventional forward combustion is a flooding process. The displacement can occur radially from the injector toward the surrounding producers. This is typically done with the wells arranged in spot patterns, for example in 5 or 7 well spot patterns in which the producers surround the injector. Alternatively, the displacement can be practised using a line drive pattern. In this pattern, the injectors and producers are arranged in alternating rows.
In these processes, the rate of combustion front advance is restricted by the oil and water in place ahead of the front. Such frontal velocities are usually low, typically in the order of 0.03 to 0.06 meters per day.
Once combustion has been initiated at the injection well, newly injected air first encounters hot sand or rock which has already been burned through. The air becomes heated by the hot sand or rock as it advances therethrough, while at the same time the latter is cooled. The hot air passes into the relatively narrow combustion zone, wherein it reacts with coke left from thermal cracking of in-place oil. In the zone just ahead of the combustion front, combustion gases, connate water and cracked volatile hydrocarbons evaporate and move ahead and form a steam bank, following which the steam and hydrocarbons condense to form a water bank and an oil bank. Beyond the oil bank, the gases flow through substantially unheated or cold reservoir toward the producers. The various zones involved in such a process are shown schematically in FIG. 1.
A known procedure for improving the thermal efficiency of a combustion process is to inject water together with the air or in alternating slugs. The water scavenges heat left in the burned out zone, is converted to steam, and transports heat through and ahead of the combustion front to provide a more efficient process.
Another known modification for combustion processes involves using oxygen-enriched air or pure oxygen instead of air as the oxidizing gas.